Coaxial directional coupler



couplers for conductors of the coaxial type.

United States Patent COAXIAL DIRECTIONAL COUPLER Edwin M. Bailey, Jr., Cambridge, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application November 30, 1945, Serial No. 631,950

4 Claims. (Cl. 333-) This invention relates to concentric coaxial directional couplers making use of the two hole wave selector principle.

A directional coupler or wave selector is a section of R. F. transmission line either of wave guide or coaxial type with a means for coupling out a constant small fraction of power traveling in the transmission line, independent of the standing wave in the transmission line. It is intended to be considerably more sensitive to power flowing in one direction than to that flowing in the opposite direction. When a directional coupler is inserted so as to couple preferentially to the wave proceeding from the transmitter, the ratio of the power deliverd to a matched load at the auxiliary line terminals to the power input to the coupler, is defined as the coupling. This is usually expressed in decibels, with the negative sign omitted for convenience. The ratio between the power appearing at the auxiliary line terminals as above and that appearing there when the coupler is turned end for end in the main transmission line (again in the absence of a reflected wave) is defined as the directivity. This quantity commonly expressed in decibels is the manner of the extent to which a directional coupler rejects the undesired wave.

The principle of the two hole wave selector or discriminator is fully described in patent application of Willard H. Fenn, Serial No. 571,319, filed January 4, 1945, now Patent No. 2,566,020 of August 28, 1951. Reference is also made to the patents of Bowen, No. 2,106,713, filed February 1, 1938, and Hansen et aL, No. 2,375,223, filed May 8, 1945.

Briefly, the rationale of operation is that the energy from one transmission line is coupled into another line through two holes separated from each other by a distance of a quarter wave length along the line. Energy will flow from the wave guide through each of the holes into the auxiliary guide and will reinforce in the original direction of travel, but will cancel out so that no energy will travel in the auxiliary guide in a direction from the holes opposite to its original direction of travel. Hence the presence of a reflected wave in the principal guide will not contribute to the energy being abstracted and led away by the auxiliary guide.

The present invention is concerned with directional Earlier forms of coaxial directional couplers consisted of two coaxial conductors with the respective outer conductors contiguous and having holes cut on the common area so that coupling took place through these holes. However, because in the coaxial mode the E lines are radial, the percentage of E lines from on coaxial conductor that can couple through the hole is relatively small and likewise the number of lines in the receiving coaxial line to take the coupling are also relatively few. Hence the coupling is very small being 00 to db or 0 to 0.3 in percent. For many uses, it is desirable that a coupling in the range 25 db to --6 db or 0.3 to 25%, or closer, be obtained. This coupling permits such percentages to be obtained from the transmitted energy by having the respective coaxial conductors inserted one within the other, the outer conductor of one serving as the inner conductor of the other with coupling apertures therebetween. Because the present invention permits all, or nearly all, of the radial E lines in the longitudinal neighborhood of a coupling aperture to couple, a much higher degree of coupling can be obtained by the invention over the earlier type for comparable sizes and numbers of the respective coupling apertures.

It is the object of this invention to provide a directional coaxial conductor coupler with the outer conductor of one portion serving as the inner conductor of that other section.

It is another object to provide a directional coaxial coupler having a wide possible range of attenuation.

Other features and objects may appear in or be suggested by the following detailed specification taken together with the drawings.

Fig. l is a sketch for illustrating the construction and principle of operation of a coupler or fixed attenuator pad in accordance with the invention providing a fixed attenuation along a single line.

Fig. 2 is a sketch illustrating similarly a different type of embodiment wherein a branch line is coupled directionally to a main line;

Fig. 3 is a section view of an actual coupler similar in principle to that of Fig. 2;

Figs. 4 and 5 are diagrams showing the diflerence between the present coupling method and that employed in the prior art.

Referring now to Fig. 1 there is shown a directional coupler for providing a fixed attenuation in a coaxial line in which it is inserted comprising an input coaxial conductor with outer conductor 2 and inner conductor 4 having the inner conductor 4 taperingly enlarged as shown at 6 to become itself the outer conductor 8 of an output coaxial conductor having inner conductor 10. To accommodate the enlarging of inner conductor 4 in the region 6, the outer conductor 2 is also taperingly enlarged as shown at 12.

Coupling between the input coaxial conductor and the output coaxial conductor is effected through two coupling holes 14 and 16 cut in the tube 8 (which will be called the intermediate conductor) which forms the inner conductor of the input line and the outer conductor of the output line. These holes are spaced a quarter of a guide wave length apart. At the end of the outer conductor 12 there is an energy absorbent termination 18 of annular shape filling in the end space between the outer conductor 12 and the intermediate conductor 8. Also there is a termination 19 at the end of the inner conductor 10 filling in the space between the inner conductor 10 and the intermediate conductor 8.

The degree of coupling with the preesnt arrangement may be chosen over a wide range by cutting away more or less of the intermediate conductor to form the coupling slots 14 and 16. The slots 14 and 16 can be cut partly (as shown) or completely through the intermediate or common conductor 8. If the slots are out completely through the common conductor maximum coupling will exist. If the slots are out completely through, polystyrene can be put into the slots to increase the amount of coupling still further and also to hold the severed center conductor together.

Appropriate tapers as at 6 and 12 permit the coaxial lines of the coupler to connect to whatever size line or fitting may be necessary. It should be pointed out that the direction of coupling is arbitrary, that is, power can equally well be coupled either from the outer to the inner coaxial line (as indicated), or vice-versa. In either case, however, the directional coupling properties are eifective. The advantages of this directional attenuation coupler as described are increased electrical coupling and mechanical compactness and also ability to withstand higher R. F. powers than many other types. Referring now to Figs. 4 and 5, the reason may be seen why the coupling is much larger in the present invention illustrated in Fig. 5 than under the prior method illustrated in Fig. 4. In a coaxial line, the fundamental mode has radial E lines between the central conductor and the outer conductor. Fig. 4 shows the prior type of coupler having the two coaxial lines with inner conductors 60 and 61 respectively, and outer conductors 62 and 63 respectively, these latter being placed parallel and contiguous to one another externally and having the coupling apertures such as 64 cut between them where they adjoin. The arrows such as 65, 66, 67 and 68 represent the E field. In this situation, only a relatively small proportion of the radially directed E lines can couple through the aperture 64. Thus lines 66 and 67 can couple but lines 65 and 68 can have no coupling effect. If the aperture is as appears in the figure, only about 20 out of the 360, then only of the available lines from the power input can couple, and furthermore, only of the lines in the driven conductor can be coupled to. The result is a very low coupling.

However, consider the situation with regard to a mutually concentric coupler in accordance with this invention as illustrated in Fig. 5. There is shown the outer transmission line composed of outer conductor 70 and intermediate conductor 71, and in turn, the inner transmission line consisting of inner conductor 72 and intermediate conductor 71. The coupling aperture 74 is cut in the intermediate conductor 71 which is the conductor common to both inner and outer transmission lines. There is here no limitation on the proportion of the whole circumference of the conductor 71 which can be cut for the coupling aperture 74. It can be small if slight coupling is desired, but it can be made larger as desired, for example about 100 as shown in the figure, or it may be if desired, cut completely around the conductor 71. In the latter supposed case, however, of course some material such as polystyrene must be used to join the two portions of the common conductor 71 which would be severed by a completely circumferential slot. And this would also of itself increase the coupling.

It can be seen that by making the slot 74 longer circumferentially that more B lines are permitted to couple and be coupled between the two transmission lines for a given longitudinal size of the hole 74. Thus all lines between 75 and 76 can couple to all lines between 77 and 78, which constitute a much greater percentage of the available E lines in the longitudinal vicinity of the slot 74. Indeed if the slot 74 is cut completely around the circumference of the intermediate conductor 71 all the input E lines will couple to all the output E lines, that is, of course, in the longitudinal region of the slots. Hence it can be seen that much greater coupling is possible under the principles of this invention than under the earlier form.

Fig. 2 shows the principles of construction of a different embodiment for directionally coupling a branch line to a main coaxial line. Here there is a main coaxial line with inner conductor 20 and an outer conductor 22 with arrows showing the assumed direction of energy travel along it from left to right. There is also a larger conductor 24 surrounding the first described line, terminated by an absorbent termination 25 at one end and at the other end by a short circuiting plate 26.

An elbow connected tube 27 is joined to the conductor 24 at a distance of one quarter wave length from the shorted end 26 and an opening provided at their intersection. The quarter wave end will produce an out of phase reflection at the intersection and so cancel out there. Within the tube 27 is a central conductor 28 and constitutes with tube 27 a branch output coaxial line. Central conductor 28 is connected to the conductor 22 which will now be referred to as the intermediate conductor.

Thus in the common region there is provided an input main coaxial line consisting of conductors 20 and 22 and an auxiliary directional coupling coaxial line consisting of conductors 22 and 24. Again coupling between the input and auxiliary lines is elfected through properly spaced apertures or slots cut circumferentially in the intermediate tubular conductor 22, so that energy travels as indicated by the arrows and will again be substantially independent of the standing wave in the main line 20-22.

Fig. 3 is a section view of an embodiment for coupling to a branch line similar in many respects to that of Fig. 2 but having a portion of a main power line outside, instead of as an inside line. That is, the main input line consists of conductors 30 and 32 having suitable directly connecting coupling flanges 33 and 34 and sockets 35 and tongue 36 for connecting it into a line (not shown) also provide with suitably corresponding fittings. Within the intermediate tubular conductor 32 is the inner conductor 38 which cooperates with conductor 32 to provide the auxiliary coupling coaxial line, which is the smaller of the two lines in overall diameter. Coupling is again effected through two circumferentially cut slots 39 and 40 disposed one quarter 9f t1 Wave length apart from each other along the intermediate conductor 32. As in the preceding embodiment the inserted output section of line 38 32 is terminated at one end by an absorbent termination 41 and at the other by a quarter wave length stub beyond the elbow 42 to a short circuit at 43. (The drawing is not in correct proportions on this detail). Similarly, there is in this embodiment branch output coaxial line consisting of the elbow tube 44 connected to the intermediate tube 32 at the elbow with a sleeve 45 for construction and broadbanding purposes, together with an inner conductor 46. A suitable direct connecting fitting for this output line is provided by members 48 and 49.

Another quarter wave termination is provided by the outer tube 50 short circuited by member 52 cooperating with the tube 44. The insulating bushing 54 is provided to position and support the inner conductor 46 and its connector extension 49.

While specific embodiments have been described and shown, it is to be understood that the principle of coaxial conductors having a mutual center axis is of broader application than the specific embodiments shown and hence, the invention is not to be deemed limited except by the prior art and the spirit of the appended claims. In particular, it is equally possible to employ any other type of directional coupling aperture to provide coupling between the internal line and the external line, in lieu of the two holes quarter Wave spaced, for example, a long longitudinally cut slot (not shown) in the intermediate conductor, with tapered ends for matching purposes. It is to be further understood that the two holes may be cut on opposite sides of the intermediate conductor. This has the advantage of compensating for any eccentricity of the center conductor, since the amount of coupling depends on the radial spacing of the slots with respect to the center conductor.

What is claimed is:

l. A coaxial directional coupler for indirectly coupling power from one coaxial line to another coaxial line comprising an outer conductor, an intermediate conductor and an inner conductor, all substantially coaxial, said intermediate conductor and said outer conductor forming one coaxial line and the intermediate conductor and the inner conductor forming a second coaxial line, said intermediate conductor having therein two circumferential apertures spaced apart one quarter wave length along it for providing directional coupling between said first forrned coaxial line and said second formed coaxial line, an annular absorbent termination at one end of said outer conductor between it and said intermediate conductor, an absorbent termination at the oppositely disposed end of said inner conductor between it and said intermediate conductor, said outer and said intermediate conductors forming said first named coaxial line being tapered suitably to permit connection to a coaxial cable or fittings therefor of any desired dimension.

2. A coaxial directional coupler for indirectly coupling power from a main coaxial line to a branch coaxial line comprising an outer conductor, an inner conductor, and an intermediate conductor, all substantially coaxial, each of said conductors having an intersecting branch, said outer conductor and said intermediate conductor serving as an input coaxial line and said inner conductor and said intermediate conductor serving as an output coupling coaxial line, said intermediate conductor having therein two directional coupling apertures spaced along said intermediate conductor a distance of one quarter wave length for coupling energy from said input line into said output coupling line, the region between said inner conductor and said intermediate conductor being terminated at one end by an absorbent termination means and at the other end by a short circuit, located one quarter wave length beyond its intersecting branch conductors, having the region between said outer branch conductor and said intermediate branch conductor terminated by a conductive member located at a quarter wave length from the branch intersection, said inner branch conductor and said intermediate branch conductor serving as an output coaxial line.

3. A directional coupling device comprising in combination, an outer conductor, an intermediate conductor, and an inner conductor, all substantially coaxial, said intermediate conductor having two circumferential apertures axially spaced an odd number of quarter wave lengths apart at the frequency of operation of said directionalcoupler for coupling electromagnetic energy between the coaxial line formed by said outer conductor and said intermediate conductor and the coaxial line formed by said intermediate conductor and said inner conductor.

4. A directional coupling device comprising in combination, an outer conductor, an intermediate conductor and an inner conductor, all substantially coaxial, and first and second apertures cut in said intermediate conductor and separated in the direction of energy propa- References Cited in the file of this patent UNITED STATES PATENTS 1,854,255 Green Apr. 19, 1932 2,239,905 Trevor Apr. 29, 1941 2,423,390 Korman July 1, 1947 2,479,650 Tiley Aug. 23, 1949 2,485,031 Bradley Oct. 18, 1949 

